1. Field of the Invention
This invention deals generally with diagnostic methods and apparatus for determining the condition and quality of bones and other materials. In particular, it discloses a method for determining non-invasively, non-destructively, ex vivo or in vivo, the condition and quality of certain bones in humans and animals.
2. Description of the Related Art
The need for assessing the mechanical properties of a material is found in a wide variety of applications. For example, it is necessary to test the strength of materials in a wide variety of industrial applications non-invasively and non-destructively. In many applications, destructive testing techniques can be utilized to determine the mechanical properties of a material. In other types of material testing, the material is easily accessible for analysis and nondestructive techniques can be utilized. Further, many materials are homogenous on a macroscopic level, permitting simplified techniques based upon assumptions of the homogeneity.
While many different techniques have been developed for investigating the mechanical properties of materials, many types of materials are not accurately analyzed using conventional techniques. For example, in many medical applications, it is desirable to determine the mechanical properties of the material for proper diagnosis and treatment. In many medical applications, however, the testing is difficult to analyze because destructive testing cannot be used, invasive types of testing are undesirable, and the non-homogenous nature of biological tissue presents unique problems. A good example is the need for accurate measurement of bone mass and bone strength as an indication of resistance to fracture and a condition for bone quality.
The capability to accurately assess resistance to fracture and the status of bone quality has great clinical significance in the diagnosis and treatment of numerous medical problems such as osteoporosis. Osteoporosis affects one in three women and one in five men over the age of 60. Over 80% of the one million fractures sustained yearly by women over the age of 50 in the United States is a consequence of osteoporosis. Half of the patients with fractures resulting from osteoporosis never recover normal functions, and 30% progress to premature death, 10% dying within three months because of peri- and post-operative complications. However, treatments exist which alter, delay or reverse the progression of osteoporosis if the disease is accurately diagnosed before fracture occurs or if the risk of fracture can be assessed. The development of improved treatments would be greatly facilitated by a technique capable of delineating their effectiveness.
In osteoporosis, bone mass is lost gradually and progressively thus altering the macro and micro bone structure and decreasing the mechanical strength of the skeleton until bone fracture due to minimal trauma results. Bones are composed of cortex (compact bone) and trabeculae (sponge bone). From the tissue level regions of bone that experience relatively high stiffness tend more towards cortical bone. Regions of bone experiencing low stiffness tend to be more trabecular. In most sites of trabecular bone, the trabecular mass is surrounded by a relatively thin layer of cortical bone which may vary in thickness between individuals. Osteoporosis is manifest as an absolute decrease in bone tissue mass primarily in the trabecular region. A person suffering from osteoporosis loses initially a greater proportional amount of trabecular bone than cortical bone.
3. Description of Current Methods
The prior art includes various approaches to measurements related to bone characteristics:
U.S. Pat. No. 4,774,959 (Palmer) purports to describe an apparatus in which a bone containing body member to be tested is placed between a pair of transducers and a predetermined sequence of tone signals having frequencies spanning a range from 200 to 600 kHz is transmitted through the body member. Amplitudes of the received signals are used to calculate the rate of change of attenuation of ultrasound transmission of the body member with respect to frequency. The rate of change of attenuation is used along with a bone width measurement in a calculation that purports to correlate reliably with changes in bone characteristics which occur with the onset of osteoporosis. Palmer does not describe the purported correlation or its reliability in any detail. Among other problems with Palmer, each single ultrasonic measurement covers a large portion of bone, such as a patient's heel. In any case, the frequencies used in Palmer would not be able to provide for measurement of a relatively small region of bone.
U.S. Pat. No. 4,913,157 (Pratt) describes similar apparatus as Palmer. Again, each single ultrasonic measurement covers a large portion of bone, such as the kneecap. From the data measured by passing ultrasound through the bone, certain ultrasound characteristics are derived, such as velocity of the ultrasound in the bone. The bone velocity of a patient is compared with a database of bone velocities, age and corresponding bone density conditions for a group of patients. Pratt purports to determine the patent's relative bone condition by comparing the bone velocity of the patient as measured to the spectrum of velocities for the patient's age. Among other deficiencies, Pratt does not describe this empirical correlation or its reliability in any detail.
U.S. Pat. No. 5,119,820 (Rossman) discloses apparatus for use in ultrasound bone density measurements in which a pair of ultrasound transducers are mounted on the arms of a caliper so that they may be placed on either side of the bone under test. From the data measured by passing ultrasound through the bone, certain ultrasound characteristics are derived, such as broadband ultrasonic attenuation. Physical properties and integrity are purportedly determined by comparing the ultrasound characteristics with a material of known acoustic properties. Alternatively, comparison may be made with a database of patients, similar to the Pratt patent.
U.S. Pat. No. 5,921,929 (Goll, et. al) discloses apparatus for use in ultrasound bone density measurements. The apparatus measures or calculates a number of “Ultrasonic Bone Indices” (“UBI”) from emitted and received ultrasound for a number of bone locations. Regions of interest in the bone are identified, for example, a region surrounding a minimum with respect to a particular UBI parameter. Certain UBIs are said by Goll to be measures of bone properties; for example, the UBI corresponding to an autoregressive moving average spectral estimation function of the stored output of the transducer is purported to be usable as a measure of bone porosity and non-connectivity.
U.S. Pat. No. 6,086,538 (Jorgensen, et al.) similarly discloses emitting and receiving ultrasonic transducers for taking ultrasonic measurements at a number of locations of a patient's bone. Various ultrasonic properties are measured or calculated, such as speed of sound transmission in the bone (SOS) or broadband ultrasonic attenuation (BUA). The ultrasonic data is said to be used to define the location of select internal anatomical features of the bone. The attenuation measures are said to be used to display an image. Jorgensen also makes a brief reference to comparing BUA and SOS values for a patient with ranges of such values for a large number of patients, similar to the database concept of Pratt and Rossman. Jorgensen does not describe this purported empirical correlation or its reliability in any detail.